A living body implant material such as artificial bone and an artificial tooth root has been receiving great attention in recent years because when the bone is broken and lost by an accident, for example, or the tooth is taken out, it can be restored by bonding the implant material or planting the implant material in the jaw bone, and thus the bone or tooth can be used in the nearly original form and a comforable life can be enjoyed. Since, however, the implant material is embedded in the living body, it is essential that the material be harmless to the human body and it must satisfy such requirements as sufficiently high strength, good processability, no dissolution, suitable specific density and good affinity to the living body.
Metals such as noble metals, alloys such as stainless steel, ceramics such as .alpha.-alumina, and in addition, apatite have heretofore been used as an implant material. These materials, however, have at least one of such disadvantages as toxicity is exhibited, the strength is insufficiently low, the processability is poor, dissolution occurs, and affinity to the living body is poor.
In order to eliminate the above disadvantages, it has been desired to develop metals or ceramics which when coated on the surface with apatite, provide a composite material having good affinity to the living body. For this purpose, a technique to bond metal and ceramic, or to bond ceramic and ceramic is needed. As such a metal-ceramic bonding technique or ceramic-ceramic bonding technique, only a plasma spray coating method has been known. Further this plasma spray coating method has disadvantages in that the yield of expensive apatite particles is low and the bonding between the coating and the substrate is not always sufficiently high. Moreover, if the plasma spray coating method is applied under too severe conditions, partial decomposition occurs during the spray coating process and it becomes necessary to conduct additional treatments such as crystallization.
In order to overcome the above prior art problems, the present inventors with another has proposed an implant material in which a metallic substrate and a coating of calcium phosphate are firmly bonded with an intermediate layer containing calcium phosphate sandwiched therebetween (Japanese Patent Application Nos. 64012/86, 64013/86 and 70504/86 (corresponding to U.S. patent application Ser. No. 29,519 filed Mar. 24, 1987)), and an implant material in which a metallic substrate and a coating of calcium phosphate are bonded together with no intermediate layer sandwiched therebetween (Japanese Patent Application No. 169547/86 (corresponding to U.S. patent application Ser. No. 74837 filed July 17, 1987)), and moreover all can be produced without the use of the spray coating method.
In these implant materials, the bonding strength between the metallic substrate and the coating of calcium phosphate is sufficiently high. However, when they are embedded in the living body, the coating of calcium phosphate having good affinity to the bone tissue may assimilate with the bone tissue, finally bringing the metallic substrate in direct contact with the bone tissue. Since the affinity of the metallic substrate to the bone tissue is poor, the bone tissue regresses, thereby degrading the bonding between the bone tissue and the metallic substrate, and in the worst case, the implant material may be rejected.